The instant invention is directed to a database input/output (I/O) control system. More specifically, the invention is directed to a database I/O system which maintains high operational efficiency during multiplex processing of transactions in a large database system.
In a conventional database I/O control system, logical I/O units use physical I/O units to perform I/O control between a buffer in a main storage unit and a database in an external storage unit. Such a system is discussed in a technical manual entitled "ACOS Software ACOS-4/AVP XR VSAS Reference Manual" published by NEC Corporation. When a request for access to information in the database is issued from a user's program, the logical I/O unit searches the buffer in the main storage unit. If, the requested information is not located in the main storage unit, the I/O unit inputs information from the database in the external storage unit and stores the information in the buffer block by block. The updating of information by the user's program is accomplished using the information in the buffer in the main storage unit. Upon completion of a transaction which forms a unit of logical processing by the user's program, the logical I/O unit uses the physical I/O unit to write back the block whose content has been updated (in the buffer) into the external storage unit.
This conventional database I/O control system has the disadvantage that processing of the next transaction by the same user's program cannot be started until the write-back operation is completed because every block updated by processing of a transaction is written back into the external storage unit when the transaction is completed.
IBM systems employ an optional background write function for database management. This function is discussed in a technical manual entitled "IMS/VS Version 1 Data Base Administration Guide" published by IBM. In this IBM technique, an updated block is sent directly to a slow external storage unit. Because the processing speed of an external storage unit is slow, either the processing speed of termination processing is reduced or the updated block must be sent to the external storage unit before termination of transaction processing.
Another disadvantage of conventional systems is that, as improvements in central processing units and main storage units lead to a significant increase in the proportion of time devoted to I/O operations, the frequency of inputting to and outputting from an external storage unit increases with a rise in the degree of multiplexing (execution of a plurality of programs in parallel using the same database), and can become greater than the processing capability of the physical I/O unit. This results in a drop in the transaction processing rate.